Synchronous satellites orbit the Earth with the same revolution rate as that of the Earth and appear above a fixed point. Synchronous satellites are useful for many applications including weather and communication applications.
These known satellites are generally gyro stabilized in orbit by spinning at a constant speed, for example, 50 rpm. The communications antenna and electronics are despun with respect to the Earth in order to maintain a fixed point of reference with a communication or ground station. In synchronous orbit, the satellite's velocity maintains it in a fixed position relative to the Earth, thereby insuring continuity in communication services.
It is generally well known in the art that various forces act on synchronous and other satellites in a manner which might move the satellite out of its stationary orbit. These forces are due to several sources including the gravitational effects of the sun and moon, the elliptical shape of the Earth, and solar radiation pressure. Other forces, such as the depletion of propellants in the fuel supply, and movements of the internal components, act in a manner to affect the balance of the spinning satellite. Such unbalance causes wobble of the spacecraft spin axis, affecting the pointing of the communication antennas. Thus, balance control mechanisms are typically provided in spinning satellites so the balance can be regulated and controlled by instructions from the Earth. Known balance mechanisms are typically provided in groups of three or four movable weights or masses, one group adapted to be translated (moved) relative to the longitudinal axis of the satellite rotor (affecting dynamic rotor balance), and another group adapted to be moved in a transverse direction on the despun structure (affecting center of gravity static balance).
Known systems typically utilize a stationary motor which rotates a jackscrew on which a movable mass is positioned. The motor is a stepper motor which has a gear assembly and a rotary potentiometer which has a reduction gear. The motor rotates the jackscrew and thereby moves the balancing mass in minute increments electrically controlled by the Earth ground station. The size of the mass and length of travel are restricted, however, with this known device. Usually, it is required that the movable mass be positioned at the center of travel during launch of the satellite, which results in the mass being placed in a cantilever condition on the jackscrew. This restricts the design in terms of travel length because the mass on the jackscrew is subjected to tremendous vibrations caused by the launch.
A second known balance mechanism also utilizes a motor, mass and jackscrew, together with a potentiometer and associated gearing mechanisms. In this design, however, the jackscrew does not rotate and the motor and mass both translate along the length of the jackscrew by means of a nut mechanism which is rotated by the motor. Although this second known balance mechanism is more efficient than the first type described above, it is also restricted in terms of size and travel length because the mass again must be at the center of travel during launch and thus must again survive the launch vibration in a cantilever configuration. In either design, it might be possible to protect against failure during launch by providing larger and sturdier jackscrews, but this would add additional weight to the satellite which is undesirable.
As a result, both of these known balance mechanisms are relatively inefficient in terms of movable mass and adjustment length compared to their total weight.